"Our fans are pretty good. They don't give away too much. Sometimes people love dropping spoilers, but our fans are good. They tend to do it in such a way that doesn't ruin it for fans who don't want to know."--Phil Keoghan

I'm assuming that with every two seasons (that are filmed back-to-back) they obviously cast both of them at the same time and have certain roles and personalities/ethnic backgrounds they want to make up the cast. Then they cast their two of each similarity and decide who will fit better for each season.